Various agricultural machines, for instance combines and balers, have choppers that cut the harvested crop, e.g. alfalfa or corn, into short pieces suitable for baling, use as ensilage, or even tedding back on the field. For some applications a chopped length of 4 cm to 5 cm is needed, for others half this length is desired. Often the same machine is used so systems must be provided to change the output chop length.
German patent document 3,141,414 of W. Silber describes such a chopper wherein the change of the chop length to preset values is effected by swinging individual or all the cutting blades from the active cutting position to an inactive position. The aligned blades are controlled by respective centrally pivoted levers pivoted on an axis fixed on the chopper's frame. These levers can hold the blades in the cutting position and can be swung back against a spring force to inactive positions in which they do not support the blades so that they can swing back of their own weight into their inactive positions. Normally even if the blades do not move by gravity, the force of the crop being pushed through the crop passage through the chopper will force the blades back. The device that controls blade position has a control shaft pivotal behind the blades about an axis parallel to that of the blades and having a plurality of cams engageable with the blade-actuating levers. The cams are differently shaped along the control shaft so that in different angular positions of the control shaft different cams act differently on the levers, with selected blades being active depending on setting. In an end position all the levers are positioned to allow the respective blades to move into the inactive positions, allowing the crop to pass through the chopper without being chopped.
The disadvantage of this system is that a separate relatively powerful drive must be provided for rotating the control shaft and its cams. Furthermore once released, the levers snap violently into the inactive or retracted position which can damage the structure. It has been proposed to provide a brake or damper for each lever to ease it into its inactive position, but such extra structure unnecessarily complicates and increases the cost of the chopper. Furthermore the mechanism is exposed to the crop and, due to its complexity, gets jammed easily so that even when released, some of the levers and/or blades remain in their active positions.
In German patent document 3,213,199 of B. Krone a chopper is described wherein the control system can set the blades directly in different positions in different arrays to produce different chop lengths. To this end support claws mounted on the selector shaft support the blades and are set parallel to the control shaft with a plurality of different support surfaces. When the shaft is rotated different support claws engage the blades to push them into different positions against the force of respective tension springs. With this system all the blades must be adjusted together, not just a few of them. Furthermore the supports for the blades are subjected to considerable stress during chopping so that the response to different forces is not clearly defined.
German patent documents 3,617,013 of K. Edlbauer and 3,644,884 of W. Lippi describe another system where each blade is pivoted at its front end on a frame of the machine and at its rear end by a coil spring set up as a toggle. Thus each spring is pivoted at one end on the respective blade and at its opposite end in a support which can be fixed on the frame of the chopper or connected via a link with a pivotal control shaft. In the latter case the number of active blades can be changed. In addition the entire frame of the chopper can be pivoted by heavy-duty hydraulic rams to set all the blades in the inactive position.
This system has several disadvantages. It is possible that, when pivoting some or all of the blades into the inactive position, the springs and blades are held in position by crop jammed into the mechanism. Thus the blades are left projecting at least partially into the feed passage-where they will cut the crop. Furthermore as a result of the relatively large coil springs the system is quite bulky and the springs in effect form a wall behind the blades that can be packed with cut crop that prevents the mechanism from functioning and makes servicing the machine, for instance to change blades, very difficult.
Finally, German patent 4,302,199 of G. Clostermeyer has a system with a plurality of vertically pivotal blades movable into an active position. They are held in the active position by support elements and are all or selectively movable into inactive positions. To this end each support element is formed by a control shaft with an end contact point engaged in the respective blade. All the control shafts are shiftable via a transverse linkage by means of a spring so that the transverse connection is formed by at least one hydraulic cylinder. Each control shaft is further connected with a controllable locking and unlocking rod for freeing or holding back the control shaft for a specific blade and all the control shafts together with their springs form emergency releases that let the blades pivot back when struck by a foreign object, such as a rock pulled into the chopper.
Here the disadvantage is that the cutting blades even when freed by their supports remain in the active cutting position because they are held in place by wet crop and miscellaneous foreign matter. The mechanism is rather bulky with its control shafts surrounded by coil springs so as to form a solid wall of structure behind the blades that fills with foreign matter and generally blocks access to the blades. Since the pivoting mechanism for the transverse beam carrying the control shafts is relatively tall, the ground clearance of this machine is also reduced excessively.